This invention relates generally to surface acoustic wave devices, and more specifically to techniques for providing such devices with acceptable long-term frequency stability.
As it is known in the art, surface acoustic wave (SAW) devices, such as resonators and delay lines, are used in a variety of applications. Generally a surface acoustic wave device comprises a pair of interdigitated transducers (IDTs), which are the input and output ports of the device, with each transducer having a set of conductive members, typically metal, disposed on or recessed within an upper portion of a surface which supports surface acoustic wave propagation.
As it is also known in the art, the thickness of conductive members comprising interdigitated transducers is determined by the desired device characteristics. The metal thickness of such conductive members will have an effect on the resonant, or center, frequency of the device and the quality factor, Q, of the device. A slight variation may also be observed in the overall losses of a SAW device as such metal thickness is varied. Based on trade-offs of these effects, conventional SAW devices have interdigitated transducers having conductive members with metal thickness ranging from 1.0% to 1.2% of an acoustic wavelength at the center frequency of the device.
As it is also known in the art, many SAW device applications require relatively constant frequency characteristics over long periods of time. The variation of frequency characteristics of a device with time is often referred to as the long-term frequency stability. Long-term frequency stability is measured in parts per million per year (ppm/year) and a typical requirement is 1 ppm/year averaged over a ten year period. The long-term frequency stability requirement of 1 ppm/year is readily met by devices which are operated at sufficiently low power levels and at frequencies below 600 MHz. However, at frequencies higher than 600 MHz a percentage of such devices will typically vary signficantly more than 1 ppm/year.
The aging mechanism, which causes the frequency characteristics of a device to vary over time, in high frequency SAW devices is not fully understood at this time. It may be activated by either acousto-migration or electro-migration of impurities into the grain boundaries between adjacent crystals of the IDTs. Acousto-migration is a phenomenon in which metal portions of a device are displaced due to mechanical vibrations and electro-migration refers to the displacement of such metal portions due to electric current. Since the cause of poor long-term frequency stability is not fully understood, such characteristics are not easily improved. Presently, in providing SAW resonator devices, a common goal is simply to separate or weed out those devices which display poor long-term frequency stability characteristics.
One technique known in the art for separating devices having good long-term frequency stability characteristics from those without such desired characteristics is to "pre-age" the devices by operating them for a predetermined period, generally on the order of several months, in order to observe any devices not having a desired long-term frequency stability characteristic. Such a process requires that each SAW device be set up in a temperature stabilized environment and its frequency frequently monitored. The delivery time of such "pre-aged" devices is greatly increased by the duration of such testing. Moreover, both the time required to monitor device frequency characteristics and the test equipment employed will significantly increase the overall cost of the devices. Additionally, in order to ship a specified quantity of devices, an additional quantity must be fabricated and tested in the anticipation of failures. The cost associated with these "extra" devices, above the shipment quantity, must also be factored into the cost of "pre-aged" devices, thereby increasing their cost. This is particularly true since often SAW devices are custom ordered with respect to frequency characteristics. Such devices which do not meet specification for aging or which are extra and do meet specification for aging nevertheless may not be suitable for sale to another customer due to potentially different specifications for frequency characteristics.